Dynamically reconfigurable motors and generators and systems with efficiency optimization

ABSTRACT

A method includes adjusting currents of a plurality of windings of a motor through a plurality of power converters coupled to the plurality of windings so that the number of poles and the number of phases of the motor are dynamically adjustable, and injecting a plurality of high-order harmonic currents into the plurality of windings of the motor through controlling the plurality of power converters to improve a performance index of the motor.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/706,718 filed on Sep. 17, 2017, which is a continuation of U.S.patent application Ser. No. 15/095,024 filed on Apr. 9, 2016, now U.S.Pat. No. 9,800,193, which is a continuation-in-part of U.S. patentapplication Ser. No. 14/467,027 filed on Aug. 24, 2014, now U.S. Pat.No. 9,490,740, which is a continuation-in-part of U.S. patentapplication Ser. No. 14/185,892 filed on Feb. 20, 2014, now U.S. Pat.No. 9,240,748, which is related to and claims priority to U.S.Provisional Application No. 61/852,335, titled, “Motor and GeneratorSystems Optimized with Power Electronics” filed on Mar. 15, 2013, whichis herein incorporated by reference.

TECHNICAL FIELD

The present invention relates to electrical drives and controls, and, inparticular embodiments, to novel motor and generator structures, and theuse of novel power electronics equipment to drive and control them.

BACKGROUND

Electrical machines (motors and generators) are widely used to processenergy and power equipment. Many of their applications require themotors and generators to operate at variable speed. Power electronicsequipment is also used to operate with the motors and generators in suchvariable speed systems, including but not limited to industrial drives,electrical vehicles, diesel-generator sets, and wind power generation.There is a strong desire to increase the efficiency of such systems,while reducing its cost and size, especially for demanding applicationssuch as electrical cars and other transportation equipment.

Unfortunately, the motor, generator and power electronics equipment invariable speed systems usually adopted standard technologies in eacharea, and are usually not optimized as a whole to achieve best results.For example, so far the vast majority of the motors and generators havea three-phase structure, and the power converters working with them areof a three-phase structure too. Significant improvement is needed tofurther optimize system performance and reduce the system cost.

SUMMARY OF THE INVENTION

These and other problems are generally solved or circumvented, andtechnical advantages are generally achieved, by preferred embodiments ofthe present invention which provides an improved resonant powerconversion.

In accordance with an embodiment, A method comprises adjusting currentsof a plurality of windings of a motor through a plurality of powerconverters coupled to the plurality of windings so that the number ofpoles and the number of phases of the motor are dynamically adjustable,and injecting a plurality of high-order harmonic currents into theplurality of windings of the motor through controlling the plurality ofpower converters to improve a performance index of the motor.

In accordance with another embodiment, a system comprises a motor havinga plurality of windings arranged into a plurality of groups, wherein allfirst terminals of windings in a group are connected to a connectionbar, and wherein each second terminal of the windings in the group iscoupled to a power converter, all power converters coupled to thewindings in the group forming a set, wherein the set of power convertersis coupled to a power source and is configured to control currents ofthe group of windings, and a controller configured to determine thenumber of poles and number of phases, and inject a high-order harmoniccomponent into the motor to improve a performance index.

In accordance with yet another embodiment, a method comprisesconfiguring a motor drive system with a motor having a plurality ofwindings arranged into a plurality of groups and a plurality of powerconverters arranged into a plurality of sets, wherein a first terminalof each winding in a group is coupled to a power converter, and whereinsecond terminals of all windings in the group are coupled to aconnection bar, and controlling the plurality of power converters toinject a plurality of high-order harmonic currents into the motor toimprove a performance index of the motor drive system.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures or processes for carrying outthe same purposes of the present invention. It should also be realizedby those skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the invention as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a three-phase drive system with a three-phase motorand power electronics system;

FIG. 2 illustrates a multi-phase drive system with a multi-phase motorand power electronics equipment in accordance with various embodimentsof the present disclosure;

FIG. 3 illustrates a control system for a multi-phase drive system inaccordance with various embodiments of the present disclosure;

FIG. 4 illustrates a slot-based drive system with a slot-configurablemotor and a slot converter based power electronics system in accordancewith various embodiments of the present disclosure;

FIG. 5A illustrates a connection end of a stator of a slot-configurablemotor in accordance with various embodiments of the present disclosure;

FIG. 5B illustrates a shorted end of a stator of a slot-configurablemotor in accordance with various embodiments of the present disclosure;

FIG. 6 illustrates a control system for a slot-based drive system shownin FIG. 4 in accordance with various embodiments of the presentdisclosure;

FIG. 7A illustrates a two-level slot power converter in accordance withvarious embodiments of the present disclosure;

FIG. 7B illustrates a three-level slot power converter in accordancewith various embodiments of the present disclosure;

FIG. 8 illustrates an embodiment of a packaging technique in accordancewith various embodiments of the present disclosure;

FIG. 9 illustrates a double-fed drive and generation system inaccordance with various embodiments of the present disclosure;

FIG. 10 illustrates a winding diagram of a 15-phase 2-pole configurationof a motor in accordance with various embodiments of the presentdisclosure;

FIG. 11 illustrates a winding diagram of a 5-phase 6-pole configurationof a motor in accordance with various embodiments of the presentdisclosure;

FIG. 12 illustrates a winding diagram of a 3-phase 10-pole configurationof a motor in accordance with various embodiments of the presentdisclosure;

FIG. 13 illustrates a motor drive system in accordance with variousembodiments of the present disclosure;

FIG. 14 illustrates a motor drive system in accordance with variousembodiments of the present disclosure;

FIG. 15 illustrates a motor drive system in accordance with variousembodiments of the present disclosure; and

FIG. 16 illustrates a motor drive system in accordance with variousembodiments of the present disclosure;

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the variousembodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

The present invention will be described with respect to preferredembodiments in a specific context, namely in a motor and motor drivesystem. The invention may also be applied, however, to a variety ofother electrical machine and machine control systems, includinggenerators, rectifiers, and inverters, and any combination thereof.Hereinafter, various embodiments will be explained in detail withreference to the accompanying drawings.

A variable speed system usually controls a motor to operate at or aroundits synchronous speed. The synchronous speed of an ac electrical machine(motor or generator) is determined by the frequency of the power supplyand the number of poles of the motor according to the followingrelationship:

${S = \frac{60f}{P}},$in which S is speed in rpm, f is the power supply frequency in Hz, and Pis the number of pairs of poles of the motor or generator.

Most variable speed applications use variable frequency in the powersupply as the main control method, but keep the pole number constant.When the speed range is wide, the frequency range is also wide.Unfortunately, neither power electronics equipment nor motors (andgenerators) are good at wide frequency ranges, and usually low frequencyoperation and high frequency operation present big challenges to thedesign of power converters, motors, and generators. This often resultsin suboptimal performance and increases cost, volume and weight of thesystem. Moreover, motors and generators above a few kW are usuallydesigned in a three-phase configuration with fixed number of poles, asis shown in FIG. 1 , where a three-phase power electronics system 110 isused to power a three-phase motor 150. A three-phase motor has a rotor,a stator, and a three-phase winding consisting three phase windings (onephase winding per phase). In high power applications, each phase has todeal with high voltage and high current, which not only increasesinsulation and cabling requirements in the motor and the system, butalso mandate parallel and/or series connection of power devices andconverters in the power electronics equipment. As power semiconductordevices cannot be put in parallel or in series without extra effort,such parallel or series connection further increases the cost and thecomplexity of the power electronics equipment.

To alleviate this problem, the number of phases in a high powerapplication can be increased, so each phase processes lower power whichcan be handled easier by power semiconductor switches. Such a system isshown in FIG. 2 , where a multi-phase power electronics system 108powers a multi-phase motor 160. The number of phase windings of 160 ishigher than 3, so each phase winding and related power electronicscomponents in 112 deal with lower power than in a three-phase system.The voltage and current in adjacent phases of an m-phase system have aphase shift of 360°/m, where m represents the number of phases in thesystem. Physically, m phase windings are evenly distributed in the spaceof a pair of poles. FIG. 2 shows a six-phase system with six windingsdistributed in a complete region of two poles. The phases are labeled A,B, C, D, E, and F. The windings of the same phase in different pairs ofpoles can be put either in series or in parallel (not shown in thedrawing), so the interconnection with the power source can be easier.FIG. 3 shows a block diagram of the control system for a multi-phasesystem. The system controller 111 provides system control function, andmay outputs system parameters such as speed, torque, power delivered tothe output etc. The output of this block is the references for the powerelectronics subsystem, which may include reference signals forfrequency, voltages, and currents. 112 is the compensation block of thepower converters. In one preferred embodiment, 112 deals with phasevariables. In another preferred embodiment, block 112 is in a DQsynchronous frame so main control variables are in dc values in steadystate. The phase voltages and currents in the power converter and themotor or generator are sensed when necessary. In a preferred embodiment,such variables are transferred into the αβ frame first through acoordinate transfer block 121 to get a general phasor presentation 124.Then through another coordinate transfer block 120 the feedback signal124 may be transferred into the DQ frame to have a DQ presentation 125.These coordinate transfers are well known in the industry. Aftercompensation in the DQ frame controller 112, control output 117, whichmay include voltage commands, are transferred into a αβ presentation 118through coordinate transfer block 113. A modulation block 114 is used togenerate PWM control signals from the phasor presentation. In onepreferred embodiment, 114 uses space vector modulation. The PWM signalscontrols the power switches in the power converter 115, which powers themotor 116. When the number of phases is high in the system, space vectormodulation may be very complex. In such a case, it may be easier to usea phasor control scheme in the modulation block 114, in which thevariables in the αβ frame can be presented as a general phasor in theform of:

V=V_(m)e^(−jθ), in which V is the general voltage vector, V_(m) is theamplitude of the vector, and θ is the angle of the vector.

From the amplitude information and angle information, the phase voltagescan be easily determined from the ideal phase relationship of phases ina multi-phase system. For example, in an m-phase system, the ith phase'svoltage can be calculated from the following relationship:

${V_{j} = {{{Vm}\;\cos\mspace{11mu}\left( {\theta - \frac{\left( {i - 1} \right) \star 360}{m}} \right)} + V_{0}}},$in which V₀ is the zero-sequence component which can be set in thecontrol system to optimize performance. Alternatively, instead of usingzero-sequence component some 3rd and/or higher order harmonic componentscan be added to the above equation to obtain better performance.

Then PWM (pulse-width modulation) signal can be used to determine theduty cycles of the switches in each phase leg. The PWM switching signalsare sent to the multi-phase power converter 115 to control the switchesin the converter 115.

In a conventional multi-phase system, the number of phases is usuallyfixed. Though the number of poles may be reconfigured in a limited rangeby different connections of winding taps, such reconfiguration isusually static. Further improvements can be made to optimize theperformance and cost tradeoffs. With an appropriate architecture of thepower electronics system and an appropriate design of the electricalmachine, in a preferred embodiment the number of phases and the numberof poles can be dynamically reconfigured during operation. FIG. 4 showsan exemplary implementation. The reconfigurable motor 170 has manyslots, labeled as S1, S2, and so on along the stator armature (but onlya portion is shown in details in the figure for the sake of clearness).In each slot, there is a one-turn slot winding. The conductor in a slotmay consist of one of multiple wires, or can be a solid piece ofconductor, such as an Aluminum or Copper bar. The winding can beinserted into the slot, or casted or molded into the slot. Also, theconductors in adjacent slots can be put in parallel, and in such casethe paralleled windings are treated as one winding in control and powerperspective. At one end of the stator, all slot windings are connectedto a connection ring, which basically shorts all slot windings on oneend to form an equivalent star connection. The connection ring may begrounded by making one or more electrically conductive contacts to themetal case of the stator or other grounded structure. The other end ofeach slot winding is connected to a slot power converter of a powerelectronics system 105, which is basically a single-phase inverter orrectifier. Depending on the system requirement, a slot power convertercan perform inverter functions, rectifier functions, or both. Because inthis architecture, a slot winding is not committed to any particularpole or phase, the phases and poles of the machine can be dynamicallyreconfigured by controlling the phase relationship of the currents andvoltages between the slot windings. By changing the phase relationshipbetween the winding currents and voltages, different poles and phasescan be established, and both the number of phases and the number ofpoles can be dynamically controlled. Therefore, a dynamicallyreconfigurable machine 170 with this embodiment structure can be calleda universal motor (or generator), and a slot-based powering architectureshould be used to fully employ the flexibility of such a machine.

FIG. 5A shows the connection end of the stator, where the slot windingsare separated. FIG. 5B shows the shorted end of the stator, where aconnection ring connects all slot windings together. A rotor ismagnetically coupled to the rotor, and has rotor windings. The detailsof the rotor windings are not shown for simplicity. In a preferredembodiment, a squirrel cage rotor winding is used, so the phases andpoles in the rotor can be automatically adapted to any statorconfiguration.

The following example shows the operation of adjusting phase and poledynamically according to this disclosure. A motor with 60 slots in itsstator may be configured to have 40 poles initially, and then each pairof poles has 3 slot windings. So initially it's a three-phase system,and the phase shift from a slot winding to an adjacent slot winding is120° electrically. When according to the system requirement the motor isconfigured to have 10 poles, each pair of poles now has 12 slotwindings. The 12 slot windings in a pair of poles can be configured as a12-phase system with the slot phase shift being 30°, or as a 6-phasesystem with every pair of two adjacent slot windings working as onephase winding and with a phase shift of 60° between adjacent phasewindings, or as a 4-phase or even 3-phase system. However, when morethan one slot wings are assigned to one phase, there will be circulatingcurrent between the slot windings, and the efficiency and powercapability is reduced. If a phase has only one slot winding, the motor'sefficiency and power capability are kept at the highest. Moreover,different pairs of poles may have different numbers of phases. Forexample, in the above 60-slot motor, a 36-pole configuration can beachieved by having 6 pairs of poles have 4 phases (in 4 slots) in eachpair, and 12 pairs of poles have 3 phases (in 3 slots) in each pair.With this uneven phase configuration, the pole number of the 60-slotexample motor can be any even number from 2 to 40, so the synchronousspeed at a given frequency can have a range of 20:1. In this way, thepole number is controlled to change in substantially fine steps, andthere are a significant large number of steps (for example the maximumpole number is higher than ½ of slot number, and the minimum pole numbermay be 2 or 4). The power rating of the motor and associated powerelectronics system can be kept constant over the whole range.

Pole number control can be a significant part of a variable-speedcontrol system. With the fine-step change of number of poles, the speedof a motor or generator can change over a wide range while the frequencyof winding currents being in a narrow range, to optimize the systemperformances. In some applications which don't need precise speedregulation, the frequency of a power supply to a winding (thus thecurrent in the winding) may be kept constant and speed control can beachieved with the change of pole numbers alone. This can result inbetter design of the power converter, especially in very high powerapplications where frequency change may be difficult or inefficient, forexample in resonant topologies. In addition, because the control of polenumber change is equivalent to a mechanical gear in changing the speed,mechanical gear boxes in a drive system can be eliminated, achievingbetter system efficiency, cost, size, and weight.

Because each slot of the universal motor may has only one winding withmodest voltage rating, the insulation requirements of the machine andassociated cabling are minimized. In addition, a low-cost manufacturingprocess similar to those used with the squirrel cage rotor in inductionmotors can be applied to the stator also. As a result, betterperformance and more power can be obtained in a machine with less cost,weight and volume. This makes the universal motors a good choice forchallenging applications such as in electrical drives in vehicles,aircrafts, ships, and other industrial applications. And a universalgenerator with this technology is also suitable in wind power generationand other outdoor energy applications.

A control block diagram is shown in FIG. 6 for a universal motor withslot power converters. The general signal flow is similar to the methodshown in FIG. 3 , but a few blocks are added and changed. The selectionof number of poles, through the pole number decision block 130, nowplays an important role, and should be made considering the speed, powerand frequency information. In one preferred embodiment, 130 is aseparate block. In another preferred embodiment, the function of theblock 130 may be part of the system controller. A pole and phase mappingblock 132 may have a mapping table to decide slot assignment, and shouldbe updated whenever there is a pole number or phase number change, sothe slots are dynamically assigned to poles and phases. To explain theoperation with the above 60-slot motor example, Slot S4 may be assignedinitially to Pole Pair P2 as Phase A in the original 40-pole three-phaseconfiguration. To move to a 36-pole configuration, a new assignmentshould be made. As discussed earlier, in a 36-pole configuration 6 pairsof poles may have 4 phases each, and 12 pairs of poles may have 3 phaseseach. Now pole pair P1 may have 4 phases, so slot S4 may be assigned topole pair P1 as Phase D. The coordinate transfers between the αβ frameand phase variables should use the information in the mapping table andbe performed individually for each pair of poles, using pair of poles asa basic subsystem. Now the windings in each pair of poles should becollectively treated as a subsystem in a pole control module, andcontrol functions related (especially the winding current control) allphase windings in a given pair of poles should be processed in thecorresponding pole control module. The blocks in a pole control moduleare denoted with italic text in FIG. 6 . Therefore, for a configurationwith P pairs of poles, there will be P pole control modules, and allpole control modules should be processed in synch. Each pole controlmodule can be treated as a thread in control software programs, and thewhole control system now becomes a multi-thread process. It will requiremore computational resources than a conventional control system as isshown in FIG. 3 . However, with the rapid progress of computerprocessing capability, the added resource requirement is not asignificant burden, and is well justified considering the advantages thedynamic pole and phase configuration can bring to the system. In thisway, the number of poles, number of phases in each pair of poles, andthe frequency of windings currents can all be controlled simultaneouslyin a well-coordinated way to achieve smooth system operation.

Every slot winding of the universal motor or generator should be poweredby a slot power converter 115. The slot power converter should be ableto control its current and voltage under any condition. Considering thedynamic change of phase and pole association a slot winding can have,it's best that the voltage and current control be self-contained to theslot power converter. Therefore, instead of using space vectormodulation which coordinates the control of all phases in a system, it'sbetter to use the phasor control scheme discussed above with FIG. 3 . Toaccomplish self-contained control, the zero-sequence component in phasevoltage or current calculation should be set to zero, and if needed some3^(rd) and/or higher order harmonic components can be added to increasethe power capability and other performance.

FIGS. 7A and 7B shows two inverter topologies suitable for the slotpower converter. FIG. 7A is a two-level converter, and FIG. 7B is athree-level converter. If high power is needed, multiple converters withthese topologies can be paralleled (preferably interleaved in control),or higher-level topologies can be used. The operation principles ofthese topologies are well known, and do not need to be discussed here.The optional filter 143 consisting of inductor L1 and capacitor C1 canbe used to reduce the current ripple and dv/dt supplied to the slotwinding, so EMI can be reduced. Other filter configuration can also beused. The dc-link circuit uses one input power source Vin. If needed twoinput sources can also be used, with one across each of the inputcapacitors Cd1 and Cd2. Part or all of the dc-link circuit 141,consisting of the input source, input capacitors and possibly otherfilter circuit, can be shared by multiple slot converters or all slotconverters in a system, to reduce the cost and complexity of the powersystem.

Due to the existence of many slots and slot converters, it's possible tooptimize system performance by disabling some of the slots and theirassociated slot converters in certain conditions. For example, inlight-load conditions, some slots can be powered off by de-activatingthe power converters (disabling the switching of power switches in theconverters) to reduce power losses and improve efficiency. The disabledslots and de-activated converters can be used as a backup system for theactive slots and converters, so the reliability and availability of thesystem can be improved. In some applications the large number ofinterconnections between the motor (generator) and the slot convertersmay present an issue. FIG. 8 shows a conceptual drawing of a packagingtechnique to alleviate the issue by minimizing the length of theinterconnections. Part or all components of the slot converters areassembled on to a substrate 160, to become an assembled package. In oneembodiment, the substrate 160 is as a printed circuit board (PCB). Thesubstrate 160 is designed to have a suitable shape and size to reduceinterference with the motor or generator operation, and is divided intomany sectors, so each sector can be easily coupled to one or more slotwindings through suitable connection means. The substrate may havecavities to allow the connection terminals of the slot windings toprotrude through. The connection terminals of the slot windings may beprocessed and shaped for easier connection. The package is coupledmechanically to the stator by connecting the connection ends of the slotwindings of the motor through the cavities of the substrate 160. Theconnection terminals of the slot windings may be connected tointermediate connectors, which are connected to the substrate. Theassociated slot converters are packaged in or near the sector. Thepackage may have several subassemblies, with each subassembly housingone or more slot converters. The circuits in the package may be cooledby one or more fans coupled to the rotor of the motor (generator), orcooled through other means.

The above described techniques can be used in both induction motor andinduction generator. For example, in doubly-fed generators found in manyapplications, the number of phase may be decided by the system to whichthe stator windings are connected to, but the number of poles may bechanged dynamically by electronic switches to reconfigure windingconnections in both rotor and stator. The switches for the rotor andtheir control circuit may be coupled to the rotor mechanically.Moreover, it may be desired to have energy storage in a power generatorsuch as in wind power applications, so the power delivered to the systemwill be more consistent and have less fluctuation with lower peaks.Traditionally, the energy storage is coupled to the system throughdedicated power processing equipment, which results in additional costs.It is desirable to integrate the energy storage with the generatorsystem to reduce system cost. FIG. 9 shows a block diagram of a systemwith energy storage coupled to the rotor windings in a multi-phasesystem (three phases are shown as an example). There may be an optionalcharger coupled between the system and the energy storage, but its powerrating would be much smaller than in a stand-alone energy storagesystem. By controlling the currents and voltages of the rotor windings,the position, strength, and rotation speed of the magnetic fieldgenerated by the rotor can be controlled, so the power flow between theenergy storage, the system, and wind source can be controlled. The polesmay be configured dynamically to change the synchronous speed tooptimize system performance over a wide speed range a wind turbine isrequired to operate. When the wind is strong, more power can be obtainedfrom the wind, and the rotor runs at a speed higher than the synchronousspeed of the stator field. The rotor should be controlled to generate afield rotating in the opposite direction of the stator field, with thespeed equal to the difference between the rotor's mechanical speed andthe stator field's synchronous speed. In this way, part of the windpower is sent to the power system electrically coupled to the statorwindings, and part of the wind power is send to the energy storage(usually batteries) coupled to the rotor windings. When the wind poweris moderate, the rotor should be controlled to rotate at or slightlybelow the synchronous speed of the stator field, so the wind power isdelivered to the system through the stator. Some power may be also drawnfrom the energy storage, and from the optional charger connected to theenergy storage if necessary. When the wind power is low, the rotor speedmay be significantly lower than the synchronous speed of the statorfield. The wind power is delivered to the power system electricallycoupled the stator, and significant power is drawn from the energystorage, and from the optional charger if necessary, through the rotorwindings and delivered to the stator windings. In this mode, as long asmore power is drawn from the wind than lost in the system, there ispositive energy gain. This allows the wind power generator to harvestmore energy than possible without energy storage. The control of polenumbers (and thus the synchronous speed of the magnetic field generatedby the stator winding currents) allows the rotor windings and theirassociated power converter to operate over a smaller frequency rangethan otherwise would be required. When the wind is too low to beutilized, energy from the storage can still be transferred to the statorwhen the rotor is stalled by proper control of the rotor windingcurrents.

This concept can also be used for other doubly-fed power generators withvariable speed, such as backup diesel generators or gas turbinestogether with battery energy storage, with their performance beingoptimized in a wide speed range, by variable number of poles andoptimized use of energy storage.

With the above described techniques, a motor can be dynamicallyreconfigured to have a big set of numbers of poles. One potentiallimitation is that each winding has only one turn which may not be ableto generate a high voltage, and the number of converters needed to powerthe motor is the same as the number of slots. As a result, many windingswith low voltages and converters are needed. For some applications thislimitation may result in an awkward design, and a different way toarrange the windings and the converters may be desired. In someapplications, a limited set of pole numbers may be able to provideacceptable system performance. In such cases the motor can be designedto have a smaller number of windings but each winding can produce ahigher voltage to achieve better results. Let's use the configurationwith lowest pole number needed (with P0 being the number of poles) asthe base configuration. With proper system design, the allowed number ofpoles can be limited to be odd multiplier of the lowest pole number,i.e. Pi=Ki*P0, where Pi is an allowed number of poles, and Ki is an oddinteger, with i being an integer as the index for the set of polenumbers. Therefore, any two slots which are 180° electrical angle apartin the base configuration are 180° apart in all allowed configurations.So, conductors in these two slots can be put in series to form awinding, and multiple turns can be easily achieved in this winding. Wecan arrange each winding in the base configuration to have a pluralityof turns placed in two slots which are 180° electrical angle apart(these two slots should be the closet pair if multiple slots are 180°electrical angle apart from one slot), and each winding is powered by apower converter. The minimum number of slots will be the least commonmultiple of all the allowed numbers of poles, or three times the maximumpole number allowed (considering at least three phases are needed ineach pair of poles in normal operation, in order to get goodperformance), whichever is bigger. In this way, multi-turn windings canbe used to optimize a motor design, and the maximum number of convertersneeded is reduced to half the slot number. By controlling thecurrents/voltages in the windings the number of poles and number ofphases of the motor can still be dynamically reconfigured duringreal-time operation. An example will be explained in the following witha motor which can be configured to have 2 poles, 6 poles and 10 poles.In this case, a 30-slots stator can be used. Fifteen windings can beplaced in the slots with each winding in two slots. Each winding isphysically coupled to a power converter, which can have differenttopologies, such as half-bridge, full-bridge, multi-level, etc. FIG. 10shows a winding arrangement for a 2-pole configuration. FIG. 11 shows awinding arrangement for a 6-pole configuration, while FIG. 12 shows awinding arrangement for a 10-pole configuration. In these drawings, thenumbers are slot numbers and represent the slots in the stator with thevalue represents the relative position of the slots on the statorarmature. An alphabetic letter represents the phase assignment for awinding. For example, A+ designates the start of Phase A winding, and B−designates the end of Phase B winding. A one-turn winding is shown ineach slot pair, but multi-turn windings can also be formed easily withinthe slot pair. In FIG. 10 , the winding in slot 2 and slot 17 isassigned to Phase I, and it starts from slot 17 and ends at slot 2. Inthe configurations shown in FIG. 11 , the same winding is assigned toPhase D, while it is assigned to Phase C in FIG. 12 . That is, indifferent winding configurations, the phase assignment of a winding maychange, but a winding is always powered by the same power converter, sothe phase relationship between different windings within each pair ofpoles can be controlled by the currents in the windings. Betweendifferent pairs of poles, the phase relationship of the same phaseshould be decided by the control algorithm. Field-oriented control canbe used to control the flux position for each pair of poles.

Other sets of poles can also be designed similarly. For example, thestator of a motor capable of operating in 2-pole, 6-pole and 14-polemodes can be similarly designed with 42 slots and 21 windings, and thestator of a motor capable of operating in 2-pole, 6-pole, 14-pole and18-pole can easily be designed with 126 slots and 63 windings.

In dynamically reconfigurable motors, it's better to design the numberof slot in the rotor to be the same as or close to the number of slotsin the stator, so the induced currents and voltages in the rotorwindings can keep up with any change in the phase and pole configurationin the stator windings relatively easily. Because a high number ofconverters and phases are usually used to power the stator windings,some power converters can be de-activated or put into standby mode, sono current flows in the windings coupled to them. This will increasesystem efficiency during light load, and also keep the system inoperation when some windings or some converters are failed. When anyconverter is de-activated, the control should be adjusted accordingly toconsider the number of phases, and number of poles change if any, so thesystem performance will not degrade significantly. Please note that themechanical speed of the rotating magnetic field generated by thecurrents of the windings assigned to any pair of poles should be thesame in steady-state operation. In any operation mode, if number of theactive slots assigned to a pair of poles is different from that of theactive slots assigned to another pair of poles, the frequency of thecurrents and voltages in a pair of poles may have to be adjusted tocompensate the airgap length difference in this pair of poles.

The above discussed technology applies mostly to dynamicallyreconfigurable induction machines (or motor) (DRIM) where a rotor canautomatically adjust to the changes of number of poles and number ofphases automatically. In principle, this is to divide the airgap orstator armature perimeter into different number of polescircumferentially according to the operation condition in a dynamicfashion. Each pair of poles has several stator slots occupied withstator windings. As is well known in the industry, a stator windingcurrent in an induction machine has two components: a magnetizingcomponent (magnetizing current) and a torque component (torque current).Assuming that in the ith pair of poles of a machine, M_(i) stator slotsare evenly distributed spatially, and stator windings are housed inthese slots, and the magnetizing currents in these windings have evenlydistributed phase angles and a local frequency off, then the movingmagnetic field generated in the airgap under the pair of poles by thesemagnetizing currents has a synchronous speed of:

S_(si)=60*f_(i)*M_(i)/M_(s) (RPM, revolutions per minute), where M_(s)is the number of slots in the stator, and the slots are evenlydistributed along the armature perimeter of the stator.

It is important to make sure the synchronous speed in each pair of poleswithin a motor is equal or very close, so a rotating magnetic field isgenerated in the whole air gap collectively by all pairs of poles in themachine, facilitating a smooth torque to be generated in the rotor. Thatis, S_(si)=S_(s), where S_(s) is the machine's synchronous speed. Fromthe above equation, it is clear that the local frequency of statorwinding currents within a pair of poles has a reciprocal relationshipwith the number of stator slots in the pair of poles. The frequency ofmagnetizing currents in a pair of poles is different from that in adifferent pair of poles, if the numbers of phase windings in the twopairs of poles. In each stator winding, the rotating magnetic field willalso generate an induced voltage at a synchronous frequency, which isequal to Ss*P/60, with P denoting the number of pole pairs. In steadystate operation, the torque current in a stator winding has the samefrequency as the induced voltage, and thus is at the synchronousfrequency, in order for a steady torque be generated by the rotor.

If the pole number of a motor is changed during an operation mode, it isbetter to change the magnetizing current frequency and the torquecurrent frequency in every affected pair of poles according to thenumber of slots in the pair of poles, so that the synchronous speed ofthe rotating magnetic field remains approximately the same before andafter the pole number change, to facilitate a smooth transient in themotor's operation.

If during an operation mode of a DCIM, some pair of poles have differentnumber of slots from another pair of poles, then the synchronousfrequency is different from the local frequency in some or all pairs ofpoles. Under this situation, the voltages and currents in some windingswill both have components in two frequencies, as the magnetizing currentwill generate a voltage in a winding due to its resistance and leakageinductance. Within a pair of poles, the phase angles of the magnetizingcurrents in all stator windings should be evenly distributed and can berepresented by a magnetizing current phasor at the local frequency, andthe phase angles of the torque currents in all stator windings shouldalso be evenly distributed, and can be represented by a torque currentphasor at the synchronous frequency. If in a pair of poles the localfrequency and the synchronous frequency are equal, the magnetizingcurrent phasor and the torque current phasor may be combined into aphase current phasor, and modeling and control of the winding currentscan follow a similar methodology as in a standard induction machine.However, if these two frequencies are different, then the magnetizingcurrent phasor and the torque current phasor need to be treatedseparately. This requires important changes in coordination transfers aswill discussed later. Of course, it is also feasible to identify a limitset of pole numbers with equal stator slots in every pair of poles, andoperate a DRIM machine within such a set of pole numbers so only, sothat the synchronous speed is always equal to the local speed in everypair of poles. For example, a motor with 42 stator slots can operatewith 14, 7, 6, 3, or 2 pairs of poles with equal slots in every pair ofpoles. During this kind of operation, the control and modelling of theDCIM motor with each number of poles can follow a similar methodology ofregular multiphase induction motors.

A machine's torque and magnetic field in an induction machine can becontrolled in a synchronous D-Q frame which rotates at the synchronousspeed. When setting up the D-Q frame for a DRIM machine where a localfrequency is different from the synchronous frequency within a pair ofpoles, D-axis currents should represent only torque components of statorwinding currents, while Q-axis currents should represent onlymagnetizing components of stator winding currents (which correspond tothe air gap magnetic field). In this way, it will be easier to controlthe torque and magnetic field in a coordinated way. For example, thedemanded torque is significantly below its maximum rating during mosttime of a motor's operation, such as when an electric vehicle iscruising at an almost constant speed. In such an operation mode, thetorque current and the magnetic field strength can be set at the rightlevels, so that the total power loss in the motor, or in the total drivesystem is minimized. There is a control freedom in this process as thetorque generated by the windings in a pair of pole is basicallyproportional to the flux linkage and the torque current within the pairof poles, so for the same torque there is freedom in choosing the fluxlinkage and torque current, which leads to a possibility of optimaldesign tradeoff. In a motor, the power losses in the windings aredetermined mainly by the currents in the windings and the resistance ofeach winding, which is a function of the frequency/frequencies of thecurrent and the temperature of the winding. The power losses caused bythe magnetic field, including the eddy current loss in the magneticmaterial (such as silicon steel sheets) and metal in a motor, is afunction of synchronous frequency, the strength of the magnetic field,and effective area of the magnetic path under a pair of poles.Therefore, it is possible to set the number of poles, which in turndetermine the synchronous frequency for a given speed, the number ofactive phases (windings) in a pair of poles, and magnetic field strengthfor a given rotor speed and torque requirements, so that the total powerloss in the power train, including power converters, motors, and otherkey components if any, to be minimized for a given mode of operation(i.e. rotor speed and torque). This may require some power convertersand windings be deactivated during some operation, and alsofield-weakening for a wide range of operations, not just at a high speedrange as in a normal system. Please note that the elimination orsimplification of mechanical gears in a DRIM motor leads to a way tooperate the motor at a lower frequency during most of its operationmode, which further reduces the power loss in the motor, and possibly inthe power converters when their switching frequency is adjustedaccordingly. To consider the complex factors affecting the efficiency ofa drive system, an efficiency model of the system under variousoperation modes may be developed and used to determine the optimumvalues for number of poles, synchronous frequency, number of activephases in a pair of poles, magnetic field strength and/or torquecurrents etc to reach an optimal or close to optimal system efficiencyfor a wide range of operations. This can significantly improve systemenergy efficiency, and thus can lead to much better battery life andmiles per charge for vehicular applications including electric vehiclesand hybrid electrical vehicles.

Moreover, completely decoupling the torque current and the magnetizingcurrent in D-Q frame allows easy coordination frame transfers betweenthe synchronous D-Q frame and a stationary frame even if the localfrequency and the synchronous frequency are not equal in a pair ofpoles. From a machine-level D-Q control variable (such as a currentreference), a corresponding control variable in the D-Q frame for eachpair of poles can be obtained by considering the number of slots (oractive slots) in the pair of poles and the number of poles in the motor,so each stator winding have equal or similar current and voltage, inorder to get better performance and avoid over stress in any winding. Ina preferable embodiment, the stator winding current control in a pair ofpole can be performed in the synchronous D-Q frame. This can beaccomplished by coordinated control among power converters associated tothe pair of poles. In another preferred embodiment, the stator windingcurrent control can be done at the converter level after transformingcurrent references in D-Q frame into stationary phasor variables. Eitherway, the frame transfers between the synchronous D-Q frame and astationary frames are required. The frame transfers between a Q-axiscurrent and magnetizing currents shall be at the local frequency for thegiven pair of poles, while the frame transfer between a D-axis currentand torque currents shall be at the synchronous frequency. Whentransferring imbalanced winding currents in a stationary frame to theD-Q frame, the D-Q components may have components at a beat frequencywhich is the difference between the synchronous frequency and the localfrequency, and at the sum frequency which is the sum of the synchronousfrequency and the local frequency, in additional to a dc component whichis the value needed from the transfer. Therefore, proper filtering maybe needed in the control system to filter out the beat frequency and thesum frequency components. With modern filters, especially digitalfilters, such a filtering task is possible. Especially, since thefrequencies are known to the control system, such information can beused in the filter design to improve the performance of the filter.Also, the frequency ranges in a DRIM drive can be made narrower andextreme frequencies can be avoided with the help of pole number changes,which also facilitate the filter design.

In a good design, it is important to eliminate or reduce the disturbancein the current and voltage of a rotor winding when it passes differentpairs of poles of the stator as the rotor rotates. This requires that ator around the boundary of two adjacent pairs of poles, the movingmagnetic field's flux density in the airgap are equal or very closeunder the two pairs of poles all the time. To achieve this goal, themagnetizing currents of in the stator windings under each pair of polesshould be controlled properly to create a smooth rotating magnetic fieldin the airgap of the machine. A good practice is to control the statorwinding currents associated with a pair of poles such that the movingmagnetic field has a substantially sinusoidal distributioncircumferentially within the pair of poles, with the amplitude the samein every pair of poles, and the initial phase angle zero at theboundaries. This can be done by proper control of the amplitude and thephase angle of magnetizing currents in the stator windings. If a pair ofpoles has any slot de-activated, then the moving magnetic field in thispair of poles will have a negative sequence component in addition to thenormal positive sequence component. A good practice in this situation isto make the positive sequence component synchronous to the movingmagnetic fields under other pairs of poles in the same machine, throughproper control of the amplitude and phase angle of currents in theactive windings. In case only two active windings are left in a pair ofpoles, the phase difference between the currents in these windings maybe 180° or 90°. In such a way, the moving magnetic fields under allpairs of poles in the DCIM motor are all in synch, and the windings inthe rotor operate in the same way as in a regular multiphase inductionmachine in steady state. Therefore, the rotor in a DCIM motor can bedesigned similarly to that in a regular multiphase induction motor, andregular squirrel cage rotors can be used in DCIM machines.

The power delivery to the stator windings (or rotor windings if they arepowered externally) can also be coordinated in the design to reduce thepower distribution cost, since the power distribution cost may berelatively high considering a high number of power converters needed ina DRIM motor. Considering the significant number of power convertersneeded to be connected to a motor, it is desirable some times to put twoor more windings in series or in parallel so the number of powerconverters and thus the number of interconnections are reduced. It mayalso be possible to properly arrange the connecting terminals ofwindings so both ends of the motor can be used for connecting powerconverters to windings, and the power converters can be packaged aroundboth ends of the motor. For example, if each slot has a conductor bar ina motor, about 50% of the conductors may be shorted at one end to acollection ring, while the rest be connected to another collection ringon the other end. As a result, both ends have connection terminals forconnections to power converters. It is also beneficial to arrange thepower converters and the windings in a motor into different groups. Inone preferred embodiment, a power converter may power a group windingwith several windings in parallel or series. In another preferredembodiment, several power converters may be grouped together to form apower converter group. FIG. 13 shows an embodiment of a techniquesplitting the power source of the power converters, and also divide thepower converters into different groups. The dc power source for thepower converters which power the windings of a motor may be from ac-dcpower converter for applications such as in most industrial application,or from batteries for applications such as in electric or hybridvehicles. The dc input power source, such as batteries in a vehicle, canbe separated into several parts, and each parts can power a group ofpower converters coupled to a plurality windings of a DRIM. In FIG. 13 ,Vs1 1301, Vs2 1311, Vs3 1321 and Vs4 1331 are input power sources whichare each a part of a dc power source. The DCIM 1370 comprises a stator1372, a rotor 1373, an air gap 1380, and a plurality of stator windingsillustrated by a winding 1371. A plurality of power converter groups1305, 1315, 1325 and 1335 are coupled between the stator windings andthe input power sources, with each power converter group powers a groupof windings. In this way, several lower-voltage input power sources canbe utilized to power the DCIM. Usually, a lower voltage power source,such as a lower-voltage battery pack, is easier to design and can bebuilt with lower cost. The currents of windings coupled to a powerconverter group or in the motor may be controlled collectively (such asin a multi-phase converter) or individually, but coordination should bemade between the winding current controls so that all winding currentsassociated with a converter group are controlled properly. It is alsopossible to connect a plurality of or all the windings associated to apower converter group together to a common point 1381 or a collectionbar 1382 at or near the motor 1370, in order to reduce the cabling tothe motor. It may also be beneficial to connect the common point orcollection bar to the power converter group (including connecting to theinput voltage source) with a group connection lead to facilitate windingcurrent control. Please note that the windings coupled to a powerconverter group do not need to be located in adjacent slots, and thepower converters associated with a given pair of poles do not need to bein a power converter group. It may be beneficial to assign the powerconverters into a power converter group in such a way that the currentflowing through the group connection lead is relatively small in allmajor operation modes. It may be also beneficial to connect the commonpoints or collection bars in a motor together in the configuration shownin FIG. 13 .

Another preferred embodiment of using split power sources is shown inFIG. 14 , where the system configuration is basically the same as inFIG. 13 , but the input power sources Vs1 1401, Vs2 1411, Vs3 1421 andVs4 1431 are put in series. A basic advantage of this configuration isthat the cabling between the input power sources and the power convertergroups can be easier, because if each input power source has about thesame voltage and provides about the same power, the currents on the twoleads coupled between a middle tap of the power source and two adjacentpower converter groups have about the same dc values but oppositedirections, so the sum of these two currents are equal to or close tozero, and only need a small cable to carry it. This allows the cost,size and weight of cabling to be reduced in the power distributionsystem, which also has switches, contactors, fuses and breakers tocontrol, protect, and manage the power delivery in an application. Forexample, in FIG. 14 , lead 1413 is the negative input power lead ofpower converter group 1415, and lead 1422 is the positive input powerlead of power converter group 1425. Both leads 1413 and 1422 are coupledto the middle tap between input power source Vs2 1411 and input powersource Vs3 1421. If Vs2 and Vs3 have about the same voltage and provideabout the same power, then the sum of currents in 1413 and 1422 is aboutzero. That is, the current on lead 1420 is very small, and can use asmall cable, wire, or bus bar in the distribution system. Therefore, thephysical layout should make leads 1413 and 1422 to be connected togetheras early as possible at a junction point, and then the junction point isconnected to the mid tap between Vs2 1411 and Vs3 1421, through lead1420 which has much smaller (say less than 60% of) conduction area thanthe combined conduction area of 1413 and 1422. Furthermore, if possibleleads 1413 and 1422 may become a single bus bar with both powerconverter groups 1415 and 1425 connected to it, so the bus bar itselfcan be small. In system design, the split input power sources may eachhave the same or close voltage rating, and each may power the same or asimilar number of wingdings. In another embodiment, the input powersources may have different voltages, but the power obtained from a powersource may be proportionally to its voltage, so the current in eachinput leads to the power converter groups may still have the same orclose amount. Then every middle tap of the power source can apply theabove discussed principle to save cabling cost and reduce the size andweight of the cabling. Again, the currents of windings coupled to apower converter group may be controlled collectively as in a multi-phaseconverter or individually, but coordination should be made between thewinding current controls so that all winding currents associated with aconverter group are controlled properly. It is also possible to connecta plurality of or all windings associated to a power converter grouptogether to a common point 1481 or a collection bar 1482 at or near themotor 1470, in order to reduce the cabling to the motor. It may also bebeneficial to connect the common point or collection bar to the powerconverter group to facilitate winding current control. However, in theconfiguration shown in FIG. 14 , windings associated with differentpower converter groups (and thus different input power sources) shouldbe isolated from each other.

The power sources are usually implemented as batteries or capacitorswhich get their energy from power electronics equipment such as ac-dcpower supplies or battery chargers. The input sources shown in FIG. 14can be powered by a battery charge as is shown in FIG. 15 . An importanttask in this case is to maintain the charge balance among the severalinput power sources in series, Vs1 1501, Vs2 1511, Vs3 1521 and Vs41531. While dedicated charge balance circuit can be used for thispurpose, the power converters coupled to each input power source canalso be configured to provide charge balance function. In one preferredembodiment, a power converter in a power converter group is controlledto work with power distribution system to bypass the input power sourceor reducing its charging current during a charging operation, when theinput source is charged fully or charging to it is not needed. This maybe implemented as turning on a plurality of switches in a phase leg, forexample turning on both S1 and S2 in the converter shown in FIG. 7A, toallow or control a dc current passing through them. All switches in aphase leg do not be in hard turn-on in this mode. Some of them could bein soft turn-on such as in a linear mode to control or limit the dccurrent. In another preferred embodiment, the converters in a convertergroups may be controlled to produce a dc current between the positiveinput lead and negative power lead during a charging phase, so thecurrent flowing into the input power source coupled to the convertergroup is reduced or eliminated. During this operation, some of thewindings in the motor may conduct a dc current, but such dc currents donot generate any torque, and thus do not otherwise interference with themotor's operation. In another preferred embodiment, a charge balancemechanism may be implemented by output more power from an input powersource with higher charge during a motor operation.

It is also possible to charge each input voltage source directly. FIG.16 shows one embodiment. In this configuration, each input voltagesource is coupled to a dedicated charging circuit. For example, inputvoltage source Vs1 1601 is coupled to a power conditioner 1607 and acoil 1606. The power conditioner 1607 may be a rectifier circuit of anac-dc converter, or a receiver circuit and a battery charging circuit ofa wireless charger. The coil 1606 may be a secondary winding of atransformer, or a receiver coil of a wireless charger. In one preferredembodiment, some of the coils 1606, 1616, 1626 and 1636 are coupledmagnetically. In another preferred embodiment, coils 1606, 1616, 1626and 1636 are receiver coils of a wireless charger, and are magneticallycoupled together and coupled to a transmitter coil of the wirelesscharger. In one embodiment, coils 1606, 1616, 1626, and 1636 may beconnected in series. In another embodiment, input power sources 1601,1611, 1621 and 1631 may be connected in series. As discussed above, withseries connection arrangement the cost, size and weight of distributioncabling can be reduced. It is also possible to connect a plurality of orall the windings associated to a power converter group together to acommon point 1681 or a collection bar 1682 at or near the motor 1370, inorder to reduce the cabling to the motor. It may also be beneficial toconnect the common point or collection bar to the power converter groupto facilitate winding current control.

The discussion so far is focused on rotating electrical machines.However, the technologies discussed can be applied to other forms ofelectrical machines, such as linear motors, with straight-forwardmodifications.

Although embodiments of the present invention and its advantages havebeen described in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the invention as defined by the appended claims.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, compositions of matter, means, methods, or steps, presentlyexisting or later to be developed, that perform substantially the samefunction or achieve substantially the same result as the correspondingembodiments described herein may be utilized according to the presentinvention. Accordingly, the appended claims are intended to includewithin their scope such processes, machines, manufacture, compositionsof matter, means, methods, or steps.

What is claimed is:
 1. A method comprising: configuring a motor with aplurality of windings evenly distributed along a perimeter such that thenumber of poles of the motor can be adjusted through adjusting currentsin the plurality of windings; controlling currents of the plurality ofwindings of the motor through a plurality of power converters coupled tothe plurality of windings so that the number of poles and the number ofphases of the motor are dynamically adjustable, and a current flowingthrough a first winding has a first phase, a current flowing through asecond winding has a second phase, and a current flowing through a thirdwinding has a third phases, wherein a phase shift between the firstwinding and the second winding is equal to a phase shift between thesecond winding and the third winding; and injecting a plurality ofhigh-order harmonic currents into the plurality of windings of the motorthrough controlling the plurality of power converters to improve a powercapacity of the motor.
 2. The method of claim 1, wherein: the pluralityof high-order harmonic currents is produced by a plurality of high-orderharmonic voltages comprising a third-order harmonic generated by theplurality of power converters.
 3. The method of claim 1, wherein: theplurality of windings of the motor is divided into a plurality ofgroups, and wherein first terminals of all windings in a group arecoupled to a connection bar, and a second terminal of each winding inthe group is coupled to one of the plurality of power converters.
 4. Themethod of claim 3, wherein: a winding of the plurality of windings is ametal bar.
 5. The method of claim 1, further comprising: controlling theplurality of power converters to continue operating the motor after oneof the plurality of windings or one of the plurality of power convertershas a failure.
 6. The method of claim 1, further comprising: determiningan injection ratio of a high-order harmonic current to a fundamentalfrequency current for improving the performance index of the motor. 7.The method of claim 1, further comprising: adjusting phase shiftsbetween currents of adjacent windings of the motor to dynamically adjustthe number of poles and the number of phases in a pair of poles.
 8. Themethod of claim 1, further comprising: injecting the plurality ofhigh-order harmonic currents into the plurality of windings of the motorto increase power capability of the motor or the plurality of powerconverters.
 9. The method of claim 1, further comprising: injecting azero-sequence component into the plurality of windings of the motor toimprove a performance index of the motor.
 10. A system comprising: amotor having a plurality of windings evenly distributed along aperimeter and arranged into a plurality of groups, wherein all firstterminals of windings in a group are connected to a connection bar, andwherein each second terminal of the windings in the group is coupled toa power converter; all power converters coupled to the windings in thegroup forming a set, wherein the set of power converters is coupled to apower source and is configured to control currents of the group ofwindings such that a current flowing through a first winding has a firstphase, a current flowing through a second winding has a second phase,and a current flowing through a third winding has a third phases,wherein a phase shift between the first winding and the second windingis equal to a phase shift between the second winding and the thirdwinding; and a controller configured to determine the number of polesand number of phases, and inject a high-order harmonic component intothe motor to improve a power capacity.
 11. The system of claim 10,wherein: two power sources coupled to two sets of power converters areconnected in series.
 12. The system of claim 10, wherein: the controlleris configured to adjust the number of poles and the number of phases ina pair of poles of the motor through adjusting currents flowing throughthe windings.
 13. The system of claim 10, wherein the controllercomprises: a pole number decision unit configured to determine thenumber of poles based on a plurality of operating parameters; a systemcontrol unit configured to generate a plurality of reference signals forcontrolling the power converters; and a modulation unit configured togenerate a plurality of PWM signals for controlling the powerconverters, wherein the modulation unit has inputs coupled to the polenumber decision unit and the system control unit.
 14. The system ofclaim 10, wherein: the controller is configured to inject a third-orderharmonic component into the motor for improving power capability of themotor.
 15. The system of claim 10, wherein: the controller is configuredto determine the number of poles based on a speed of the motor.
 16. Amethod comprising: configuring a motor drive system with a motor havinga plurality of windings coupled to a plurality of power converters,wherein the plurality of windings is distributed evenly along aperimeter and arranged into a plurality of groups and the plurality ofpower converters is arranged into a plurality of sets, and wherein afirst terminal of each winding in a group is coupled to a powerconverter, and wherein second terminals of all windings in the group arecoupled to a connection bar; and controlling the plurality of powerconverters such that a current flowing through a first winding has afirst phase, a current flowing through a second winding has a secondphase, and a current flowing through a third winding has a third phases,wherein a phase shift between the first winding and the second windingis equal to a phase shift between the second winding and the thirdwinding, and a plurality of high-order harmonic currents is injectedinto the motor to improve a power capacity of the motor drive system.17. The method of claim 16, wherein: the connection bar has a ringshape.
 18. The method of claim 16, wherein: the number of poles of themotor and the number of phases in a pair of poles are configured to bedynamically adjusted during operation of the motor drive system.
 19. Themethod of claim 18, further comprising: configuring the motor drivesystem such that the motor has a first number of poles at a first speedand a second number of poles at a second speed lower than the firstspeed, wherein the second number is greater than the first number. 20.The method of claim 16, further comprising: injecting a third-orderharmonic current into the motor to improve the power capability of themotor drive system.